ОПИСАНИЕ

The LT3905 is a fixed frequency current-mode step-up converter designed to bias avalanche photodiodes (APD) in optical receivers. The LT3905 features high side APD current monitoring over four decades of dynamic range with better than 2% relative accuracy over the 3µA to 3mA range.

The maximum APD current is programmed with a single resistor, and a fast current limiter with indicator protects the APD during overload conditions. Adjustable output voltage provides dynamic bias control, and an adjustable loss-of-signal indicator flags low APD current.

Coupled with the integrated DMOS power switch and Schottky rectifier, the LT3905 provides a compact total solution with few external components and low solution cost. Constant switching frequency results in predictable output noise that is easy to filter.

The LT3905 is available in the tiny footprint (3mm × 3mm) 16-Lead QFN Package.

 

ОСОБЕННОСТИ

65V, 350mA Internal DMOS Switch

Integrated Schottky Diode

2% Accurate APD Current Monitoring Over the 3µA to 3mA Range

Single Resistor Programs Maximum APD Current

Fast APD Current Limiter with Indicator

Programmable Loss-of-Signal Indicator

CTRL Pin Adjusts Output Voltage

Programmable VIN Undervoltage Lockout

High Efficiency Step-Up Converter

Selectable 1MHz/2MHz Switching Frequency

Internally Compensated

Internal Soft-Start

2.7V to 12V VIN Range

Low Shutdown Current: <1µA

 

ПРИЛОЖЕНИЯ

APD Bias

PIN Diode Bias

Optical Receivers and Modules

Fiber Optic Network Equipment

 

ОПЕРАЦИЯ

Обзор

The LT3905 combines a 65V current mode step-up DC/DC converter with a combination APD current monitor and regulator to provide accurate monitoring and efficient supply while offering additional flexibility and protection.

The step-up converter utilizes a 65V, 0.75Ω DMOS power switch and integrated Schottky diode to convert a low input voltage to a higher voltage appropriate for powering the APD. The operation of the step-up converter is a fixed frequency, current mode topology with internal compensation and accurate current limit.

 

APPLICATIONS INFORMATION

Switching Frequency

The LT3905 allows selection between 1MHz and 2MHz switching frequency. Tie the fSEL pin to a voltage greater than 0.9V to select 2MHz mode, or tie to GND to select 1MHz mode.

Обратная связь по напряжению

The LT3905 error amplifier is equipped with both a fixed internal reference of 1.248V and an adjustable external reference input (CTRL). This feature allows users to select between using the built-in reference and an external reference voltage when programming the output voltage. It is possible to adjust the voltage at the CTRL pin while the device is operating, in order to alter the output voltage of LT3905 and therefore the bias voltage of the APD.

INDUCTOR Selection

The inductor used with LT3905 should have a saturation rating of 400mA or greater. If the device is used in an application where the input supply is hot-plugged, the saturation current should exceed the peak inrush current.

APD Current Monitor Transient Response

The transient response of the APD current monitor is a key performance characteristic. It is essentially a function of the signal levels, since the small signal bandwidth increases with the input signal.

At greater than 10µA, the LT3905 APD current mirror typically has several hundred nanosecond response time. To measure such fast transient response, a wideband transimpedance amplifier is implemented using the LT6210 as shown in Figure 6. Operating in a shunt configuration, the amplifier buffers the MON output and dramatically reduces the effective output impedance. Note that there is an inversion and DC reference offset in the final measurement.

Layout Hints

The high speed operation of the LT3905 demands careful attention to board layout. You will not get advertised performance with careless layout. To prevent radiation and high frequency resonance problems, proper layout of the high frequency switching path is essential. Keep the output capacitor as close to the Schottky diode (VOUT pin) as possible. Minimize the length and area of all traces connected to the switch pin, and always use a ground plane under the switching regulator to minimize interplane coupling. The signal path including the switch, output diode and output capacitor contains nanosecond rise and fall times and should be kept as short as possible.